1. Field of the Invention
The present invention relates to an apparatus and a method for an exposure technique in the photolithographic process, such as projecting a pattern through a projection optical system and transferring the pattern on a substrate, to manufacture advanced solid state devices such as semiconductor elements, image pick-up devices (e.g. CCD), liquid crystal display elements and thin film magnetic heads.
2. Background Art
Photographic exposure technology constitutes a nucleus of semiconductor device fabrication, and as the device circuits become more highly integrated, it becomes crucial to improve resolution capability and transfer fidelity of images produced in the optical systems. Typical photolithographic process involves such steps as application of a resist film on a substrate, exposure and development of the resist patterns. To improve image resolution and transfer fidelity, it is critical to provide a precise control over the exposure dose for an optimal degree of exposure of the photo-resist film applied to a wafer as the substrate.
Conventionally, a general practice in semiconductor fabrication plants has been to use, as exposure light, i-line of wavelength at about 365 nm produced by a mercury lamp, in a so-called step-and-repeat exposure arrangement including a stepper for image projection at an image reduction ratio of ⅕ from the reticle to the wafer. In recent years, as larger size wafers have become more common, the size of the projected pattern image have also increased, but in order to avoid excessively large projected image size, a different technique of exposure is receiving attention. This technique is called step-and-scan method, which involves a movable reticle within the field of view of the object plane in a certain direction at a constant speed, and a corresponding movable wafer within the field of view of the image plane to be moved at the same relative speed as the reduction ratio so that the overall image of a circuit pattern on the reticle can be transferred onto each region of the wafer surface.
In the conventional methods of exposure dose control, it is assumed that the transmission coefficient of a projection optical system with respect to an illumination light does not vary in the short period involved in an exposure process, so that a light meter on the wafer stage is considered to be adequate for exposure control purposes. For example, exposure on the wafer surface is determined by measuring the transmission coefficient of the projection optical system just prior to the exposure event, measuring the quantity of illumination light in a branched path of the illumination optical system, and computing the exposure necessary according to the measured values of transmission coefficient and illumination dose. In the stepper system which is static, exposure duration is controlled so that the accumulated value of the calculated amount of exposure dose will reach a certain value, and in a scanning system which is dynamic, the output power or scanning speed is controlled so as to maintain the calculated quantity of exposure dose (for a given illuminance) at a constant value.
In recent years, there has been a trend towards using shorter wavelengths for the exposure light to improve the optical resolution, such that ultraviolet light from an excimer laser source is used in some step-and-repeat and step-and-scan apparatus. There is a serious move in some production lines to use a KrF excimer laser emitting at 248 nm, and there has been promising developments for wavelengths shorter than 200 nm, such as ArF excimer lasers generating ultraviolet pulses at 193 nm.
However, ultraviolet pulses produced from ArF excimer laser contain several oxygen absorption bands within the wavelength band in their natural oscillation states, such that, to use the laser as an exposure pulse source, it is desirable to restrict the light spectrum to a wavelength band that avoids such absorption bands. Furthermore, it is desirable that the illumination path (between light source and reticle) and the projection path (between reticle and wafer) be as free of oxygen gas as possible, in other words, it is desirable that most of the illumination path and projection path be surrounded by an inert gas environment (nitrogen or helium gas). Examples of projection exposure apparatuses based on such an ArF excimer laser light source have been disclosed in Japanese Patent Applications, First Publications, Hei 6-260385 and Hei 6-260386 (corresponding to U.S. Pat. No. 5,559,584).
Presently, there are only two optical grade materials available commercially that are known to produce a relatively high transmission for such ultraviolet pulses generated by excimer lasers (especially for wavelength shorter than 200 nm); they are quartz (SiO2) and fluoro spar or fluorite (CaF2). Other such materials include magnesium fluoride and lithium fluoride, but for use as optical material for projection exposure apparatus, there are still unresolved problems of fabricability and durability.
Other associated concerns are optical projection systems for use in projection exposure apparatus. There are dioptric system (refractive) and cata-dioptric systems which combine refractive elements (lenses) with reflective elements (specially concave mirror). Regardless of which type of projection system is used, if refractive elements (transmissive elements) are involved in some parts of the system, one is forced to use at least one of either quartz or fluorite, at the present time. Furthermore, whether refractive or reflective, these optical components are used with multilayers of vapor-deposited surface coatings such as anti-reflection or protective coatings, and they are manufactured to fulfill custom specifications of certain optical components. The particular property of interest in this case is the magnitude of absolute transmissivity of lenses or absolute value of reflectivity of the optical elements made from these optical materials.
For example, for a single lens element, incident surface and exiting surface are generally both coated with anti-reflection coating so as to increase transmissivity as much as possible. In precision imaging systems such as projection exposure apparatus, there are twenty to thirty lenses to provide compensation for various aberration effects, such that, even if the transmissivity of each lens element is only slightly less than 100%, the overall transmissivity of the projection system is decreased considerably. Same is true for a projection exposure apparatus using some reflective elements so that if each reflective element has low reflection, the overall transmission of the projection system is decreased greatly.
For example, if there are twenty-five lens elements in the focusing path of the projection system, if each lens has a transmission coefficient of 96%, the overall transmission coefficient xcex5 becomes fairly small at about 36% (≈0.9625xc3x97100). When the transmission coefficient of the projection system is low, it is necessary to either increase the luminocity (energy) of exposure light for projecting the circuit pattern on the reticle on the wafer, or use a ultraviolet-sensitive photoresist of higher sensitivity, otherwise the longer exposure required would reduce productivity. Therefore, practical solution for the projection exposure apparatus is to use an excimer laser of higher output power.
However, as trials progressed using such excimer lasers having relatively larger projected field size, a new phenomenon was discovered that, in relatively short time, the transmission coefficient of coating materials on the optical elements (e.g., anti-reflection coating) underwent dynamic changes due to the use of ultraviolet pulses (from KrF or ArF excimer lasers). It has been discovered since that the same phenomenon can occur not only on optical elements in the projection path but also on those in the illumination path for reticle (quartz plate) as well as on the reticle itself.
Such a phenomenon is considered to occur because the performance of optical components are affected by the impurity effects, such as particles adhering to the optical surface or particles floating in the optical paths. Such impurity particles may occur naturally in the gas present in the space in the projection path or illumination path (air, nitrogen gas and the like), or they may be organic molecules generated from adhesives used to cement optical elements to the lens barrel, or they may be impurities (such as water and carbohydrate molecules, or other light dispersive substances) released from inside surfaces of the lens barrel (anti-reflection coating). The result is that operational difficulties are caused by relatively large variations in the transmission coefficient of projection system or illumination system. Such difficulties are a result of decrease in the transmission coefficient due to adhesion of particles but, conversely, they can also be caused by vaporization of materials on the surface of optical elements by strong ultraviolet irradiation to result in increasing their transmission coefficient. Whether the transmission coefficient would be decreased or increased by impurity effects is dependent on a number of system configuration factors such as the position of the optical elements, method of attachment and the nature of the gas surrounding the elements.
For example, in the above example of a 25-lens projection system having a transmission coefficient xcex5 of 36%, if the transmission coefficient of each lens is decreased by 1%, the overall transmission coefficient xcex5 of the system drops to about 27.7% (≈0.9525xc3x97100).
Such a variation in transmissivity would interfere with providing an optimum exposure dose at the wafer, accompanied by a danger of degradation in the transfer fidelity of fine patterns of design line widths of the order of 0.18xcx9c0.25 xcexcm. As disclosed in a Japanese Patent Application, First Publication, Hei 2-135723 (corresponding to U.S. Pat. No. 5,191,374), conventional projection exposure apparatus typically relies on measuring the optical power at a given point in the illumination path of the system, and, based on the measurement results, adjusts the pulse power (energy per pulse) of the pulses generated from the excimer laser to arrive at an optimum exposure level. For this reason, the existing method does not consider the effects of variance in transmissivity in the illumination and projection paths, past the monitoring point in the illumination path, thus leading to inaccurate estimate of exposure dose control. Furthermore, such variations in the transmission coefficient of the system is likely to lead to changes in optical property of the projection optical system.
It has further been found that, when irradiation of ultraviolet pulses is stopped, the system exhibits a phenomenon of gradual recovery (change) of transmission coefficient. In such a case, upon resuming exposure by ultraviolet pulse radiation, there is a danger of improper exposure caused by undetected changes in the system transmission coefficient that have taken place in the projection exposure apparatus.
It is a first object of the present invention to provide a projection exposure apparatus that enables to accurately update the changes in the transmission coefficient of the system during the exposure process.
It is a second object of the present invention to provide a projection exposure apparatus that prevents a degradation in exposure dose on the substrate, caused by changes in illuminance (or pulse energy variation) at the substrate brought about by changes in the system transmission coefficient.
It is also an object of the present invention to provide a method of operating the projection exposure apparatus of the present invention.
The first projection exposure apparatus having an illumination optical system for emitting an energy beam, for example, of wavelengths in an ultraviolet region, and illuminating a pattern formed on mask, a projection optical system for projecting an image of the pattern onto a substrate and a substrate stage for positioning the substrate; is comprised by: a beam energy measuring system for measuring input energy of the energy beam input in the projection optical system; a referencing member, fixedly disposed on the substrate stage, having a plurality of reference marks, to correspond with alignment marks formed on the mask, and a window section for transmitting or reflecting the energy beam; a detector device for detecting illuminance of the energy beam passing through the window section; an alignment sensor for detecting a positional deviation between the alignment marks and the reference marks; and a computation system for computing a transmission coefficient of the energy beam through the projection optical system, according to result output from the beam energy measuring system and detection result output from the detector device.
Accordingly, the present scanning exposure apparatus allows quick checking of transmissivity of the system, during exchanging of mask or substrate for example, to track any change in the optical integrity of the system. The alignment sensor is used to measure a positional deviation between the alignment mark on the masking device and the reference mark on the referencing member to check misalignment of the masking device with respect to the substrate stage. Nearly simultaneously, exposure dose passing through the window section is measured by the detector device to determine the exposure dose radiating on the substrate and the transmission coefficient through the projection optical system and the window section. Therefore, every time the wafer is exchanged or after a certain number of wafers have been exposed, it is possible to efficiently monitor system transmissivity by frequent computations of the system transmission coefficient, without having to provide a separate transmission measuring step.
Changes in the transmission coefficient thus monitored is used first to control the exposure dose at the target value. Further, the results can also be used to correct reflection coefficient measurement results from the substrate and for calibrating the sensor for checking the transmissivity of the projection optical system from outside the exposure region. Also, during preliminary setup adjustments of the apparatus, performance characteristics of transmission coefficient in the projection optical system (illumination history), can be determined as a function of cumulative exposure time, and at the time of production operation, expected change in the transmissivity of the apparatus may be corrected according to such a pre-determined function. In this case, higher precision of exposure control can be attained by successively correcting such function with measured actual change in the transmissivity during wafer exchange and other breaks in the process.
According to a second aspect of the present invention, the projection exposure apparatus having an illumination optical system for emitting an energy beam, for example, of wavelengths in an ultraviolet region and illuminating a pattern formed on mask, a projection optical system for projecting an image of the pattern onto a substrate and a substrate stage for positioning the substrate; is comprised by: a referencing member, fixedly disposed on the substrate stage, having a plurality of reference marks, to correspond with alignment marks formed on the mask, and a window section for transmitting or reflecting the energy beam; a detector device for detecting illuminance of the energy beam passing through the window section; an alignment sensor for detecting a positional deviation between the alignment marks and the reference marks; and a luminosity control system for controlling exposure dose radiating on the substrate from the illumination optical system through the projection optical system, according to result output from the detector device.
Accordingly, during exchanging of mask or substrate for example, the detector device is used to measure the exposure dose passing through the projection optical system. Therefore, illuminance at the wafer surface, which includes changes in transmissivity of the projection optical system, can be monitored accurately. Thus, by controlling the exposure dose according to the monitored results, degradation in exposure dose control, brought about by changes in illuminance (or pulse energy change) on the substrate caused by changes in transmission coefficient in the projection optical system, can be prevented.
An example of the energy beam is a pulse light of less than 200 nm wavelength, and a mask stage for moving the masking device is provided, and during a process of scanning exposure, the substrate and the mask are scanned under the projection optical system by translating the substrate stage synchronously with the mask stage. This arrangement means that the scanning exposure apparatus is an apparatus based on step-and-scan method.
A method of using the scanning exposure apparatus of the present invention includes a step in which, using the alignment sensor and nearly concurrently detecting a positional deviation of the alignment marks with respect to the reference marks, illuminance of the energy beam is determined in cooperation with the window section and the detector device. As explained already, transmissivity (transmitted exposure dose) can thus be tracked accurately without sacrificing productivity of the photolithographic production line.